Remove the root part of the celeriac. Rub the celeriac with oil and salt. Stick a thermometer into the center of the celeriac and bake at 180°C until a core temperature of 90°C (about 1 ½-depending on the size). Remove the peel with a knife and cut the celeriac into small irregular pieces (3-5 per person) Keep the remaining pieces and the peel for the purée and the broth.
Brown the butter. Add the pieces of celeriac and simmer for 5 minutes. Drain the celeriac and mix in a blender with some of the brown butter and a small amount of water. Keep the rest of the butter for the sauce. Season the purée with salt and pour it into a piping bag.
Vacuum seal the celeriac with the water and the salt . Cook in a steam oven at 100°C for 20 minutes. Strain the broth and reduce it. (If you don’t have access to a steam oven you can let the broth simmer in a sauce pan instead.
Reduce the whey until 10% remains. Add the remaining butter from the celeriac and the truffle juice. Add apple purée, celeriac broth and salt to taste.
Marinate the mushrooms in some olive oil and flash them quickly with a blowtorch. Season with salt an split the mushrooms lengthwise. Just before serving, slice the apple thinly on a mandoline and cut out small circles (ø 2,5cm). Roast the hazelnuts and crush them with a knife. Cut the cress and grate the truffle over the plates.